METHOD OF PRODUCING AN ARTICLE WITH A BODY OF GLASS HAVING PROTECTIVE COATINGS OF POLYMERIC MATERIAL

Abstract: In a method of producing a glass container (2) having a projective coating, at least one layer (5, 6) of elastic polymeric material, such as styrene rubber and polyurethane, and a friction-reducing coating (7), preferably of polyethylene, are successively applied to the glass container. In one variant, a special friction-reducing agent is admixed to a polyurethane layer (6). The friction-reducing agent consists of a wax emulsion containing ethylene-bis-stearamide.

Description: METHOD OF PRODUCING AN ARTICLE WITH A BODY OF GLASS HAVING PROTECTIVE COATINGS OF POLYMERIC MATERIAL This invention relates to a method of producing an article of glass, especially a glass container, having a protective coating of poly¬ meric material. The invention further concerns an article of glass produced according to this method, as well as the use of a special agent for producing the coating on the article of glass. As is well known, glass containers are advantageous in many ways. Thus, the glass is absolutely impervious, does not attract tastes or smells to it, and usually is easy to recycle. However, glass con- tainers are also brittle and are often felt to be unnecessarily heavy, not least by the consumer. Furthermore, the glass surface is easily scratched or otherwise damaged, which may drastically reduce the strength of the glass. In view of these drawbacks, efforts have been made to develop containers with a body of glass, to which are applied two protective coatings of polymeric material. The glass is first coated with an elastic layer of rubber, which absorbs impacts and keeps fragmented glass together if the container should break (known in the art as fragment retention). The rubber layer also protects against ultraviolet radiation. A more resistant outer layer, which is to protect against wear and scratching, is then applied to the rubber layer. Due to the pro¬ tective polymer coating, the weight of the glass container can be much reduced while maintaining the strength of the glass. US-A-4,053,076, for instance, discloses the coating of glass with such polymer layers. A brochure entitled "Det nya glaset" ("The New Glass"), publish¬ ed in February 1993 by the PLM Glass Division, describes a similar technique. Thus, a returnable glass bottle is coated with two protec¬ tive polymer layers. The inner layer essentially consists of styrene- butadiene rubber (SBR), which has good qualities of shock absorption and retention, while the outer layer essentially consists of polyure¬ thane (PU), which primarily inhibits wear. To improve the adhesion of the SBR layer, a primer coating is preferably applied to the glass surface as a first step. The primer coat¬ ing consists of metal oxide, such as stannic oxide. If the bottles are to be handled in any way between the application of the primer coating and the application of the polymer layers, an additional coating of organic material is preferably applied to the primer layer. This addi¬ tional coating, for instance consisting of a fatty acid ester, then serves to reduce the friction between the bottles as they are handled in the production line. However, also the coating technique described above has its drawbacks, the most serious being the fact that the outer PU layer results in too high a friction between the bottles as these are handled in the filling machines and washing installations commonly employed when recycling the bottles. In tests, the coefficient of friction between two bottles subjected to a load of, respectively, 5 kg and 30 kg (dry and wet surfaces, respectively) has been found to be 0.3-0.6. In order to be acceptable, the coefficient of friction should be much lower, pre¬ ferably in the range of 0.1-0.2 or, which is even better, lower than that. For comparative purposes, it may mentioned that the corre- sponding values for the coefficient of friction between two untreated glass surfaces are about 0.6-0.8. A further drawback is that the lye solution used for washing the bottles may have an adverse effect on the protective qualities of the PU layer. In order to reduce the friction, articles of glass are sometimes coated with a very thin polymer layer, optionally with an intermediate primer coating of metal oxide, for instance stannic oxide, in which case coefficients of friction as low as 0.1-0.2 have been obtained. If there is no preceding primer treatment, the adhesion of the polymer coating will, however, be imperfect, and the polymer coating will thus be worn off or washed off when the articles of glass are cleaned in a basic environment, which constitutes a compulsory step in the recycl¬ ing process. Although the provision of a primer coating improves the adhesion, it only takes 3 or 4 washings before the protective coatings have been worn and/ or washed off and the friction increases. Thus, this known method of reducing the friction is unsuitable in view of today's high requirements on recycling, according to which glass bottles should be recyclable 20-30 times while retaining their strength and a sufficiently low friction. DE-A-24 56 318 discloses a further instance of background art, in which a protective coating applied to an article of glass consists of polyethylene in one embodiment. Also, GB-A- 1 ,347, 655 describes the application of an outer coating of polyester or epoxy plastic to a rub¬ ber layer that has been applied to a glass bottle. Furthermore, US-A- 3,920,869 describes the application to a glass container of a first coating of metal oxide, to which is applied an additional coating of fatty acid, to which is finally applied an outer coating of e.g. polyethy¬ lene. One object of the invention is to remedy the above drawbacks by providing a method of producing an article of glass , especially a glass container, which is equipped with an improved protective coating of polymeric material and has a low coefficient of friction. Another object of the invention is to provide a returnable glass container which, with retained or increased strength and a low coef¬ ficient of friction, can be refilled at least 20 times after being washed in lye solution. A further object of the invention is to provide an article of glass and a use thereof remedying the above-mentioned drawbacks of the background art. According to the invention, these and other objects, which will appear from the following description, have now been achieved by the methods that are generally defined in appended claims 1 and 2. Pre¬ ferred modes of implementation are stated in appended subclaims 3-6. Also, these objects are attained by the article of glass and the use thereof defined in appended claims 7 and 8, respectively. A first aspect of the invention in accordance with appended claim 1 provides a method of production, in which a number of diffe¬ rent coatings and layers are applied to the article of glass in such a manner as to be strongly bonded to each other and to the glass. If such a protective coating is applied to a glass container, the latter will be well suited for recycling, since this coating protects the glass sur- face against damage, enables a lighter glass container, reduces the degrading effect the ultraviolet light has on the contents of the con¬ tainer, and in addition is highly resistant to repeated mechanical wear and the effects of a basic environment, because the low friction on the outer surface can be maintained. Moreover, the protective coating has a retentive effect if the glass container is crushed, which helps to pre¬ vent injuries, e.g. cuts caused by fragmented glass. A second aspect of the invention in accordance with appended claim 2 provides a method of production, in which a special friction- reducing agent is admixed to the elastic polymer layer to which the outermost friction-reducing coating is applied. This combination of two friction-reducing measures results in a protective coating having a very low coefficient of friction in the order of 0.1, as desired. The invention will be described in more detail below with refe¬ rence to the accompanying drawings of non-restricting embodiments. In the drawings, Fig. 1 shows a glass bottle produced in accordance with the invention; Fig. 2 is an enlarged, schematic partial section of the wall of the glass bottle; Fig. 3 shows how two glass bottles are tested as to friction; Figs 4-6 show different curves for the coefficient of friction of a glass bottle provided with a projective coating according to the inven¬ tion; Fig. 7 is an exploded view of the coatings and layers that are applied to the glass in accordance with the invention; and Fig. 8 shows a curve illustrating the pressure strength when a PU coating is provided. Fig. 1 shows a glass bottle 1 that has been produced according to a first variant of the inventive method. Fig. 2 is a partial section illustrating the coatings and layers 3-7 applied to the body 2 of the bottle 1. These coatings and layers will be described in more detail below. It should be emphasised that Fig. 2 is but a schematic view, which does not show the thicknesses of the coatings according to scale. In a currently preferred mode of implementation of the above- mentioned variant of the invention, the glass bottle 1 is produced as follows. First, a coating 3, e.g. of stannic oxide, is applied to the glass body 2 at a temperature of about 450-650°C. The coating 3 is applied in an amount of about 3-10 μg/cm 2 and to a "thickness" of up to about 0.05 μm. In actual practice, the bottle 1 is conveyed through a station where a gaseous stannic compound is present in the form of a mist that reacts with the glass surface to form the coating of stannic oxide 3. The coating of stannic oxide 3 forms a so-called primer, which adheres strongly to the glass surface and to which an additional layer of polymeric material may be applied. The coating 3 consists of some suitable metal oxide, such as stannic oxide or titanium oxide. Natu- rally, other primer materials are also conceivable. After cooling to about 40-150°C, a coating 4 is applied to the coating of stannic oxide 3. The coating 4 contains organic material having a friction-reducing effect, for instance a fatty acid ester such as polyoxyethylene glycerol fatty acid ester, which is applied in an amount of about 0.1- 10 μg/cm 2 and to a "thickness" of up to about 0.1 μm. The coating 4 may alternatively consist of some other organic material, such as unsaturated fatty acids or the like. The coating 4 serves to reduce the friction to such an extent that the bottles can be handled in the production line without risking any bad scratches or similar defects before the application of the pro¬ jective polymer layers. After further cooling to about 20-50°C, a layer 5 of an aqueous emulsion containing poly(styrene/ butadiene), in the following referred to as styrene rubber (SBR), is applied to the coating 4 to a thickness of about 40-120 μm. The SBR layer 5 is dried at a temperature of about 80-120°C, preferably about 100°C. The shock-absorbing and retentive layer 5 is produced by immersing the rotating bottle 1 in a bath that contains SBR in aqueous emulsion, whereupon the bottle is dried. After further cooling to about 20-60°C, a layer 6 of polyurethane (PU), preferably based on polyether, is applied to the SBR layer 5. The wear-inhibiting PU layer 6, which has a thickness of about 20-60 μm, is produced by applying an aqueous emulsion containing PU to the SBR layer 5, whereupon drying is performed at a temperature of about 80-120°C, preferably about 100°C. Also in this case, the bottle is immersed in a bath that contains PU in aqueous emulsion. Thereafter, the SBR and PU layers 5, 6 are jointly cured at a temperature of about 160-190°C, preferably about 170- 180°C. After cooling to about 60- 150°C, a final, outer coating 7 of polyethylene (PE) is applied to the PU layer 6. The PE coating 7, which is applied in an amount of about 0.1-10 μg/cm 2 , is produced by applying an aqueous emulsion containing polyethylene to the PU layer, whereupon drying is performed. The "thickness" of the coating 7 is approximately 0.01-0.1 μm. Tests have shown that there is excellent adhesion between the PE coating 7 and PU layer 6. Furthermore, the PE coating 7 has been found to produce a coefficient of friction of about 0.1-0.2, which remains essentially unaltered for a great number of refills (about 20-30) involving washing of the bottle in lye solution. Fig. 3 shows how two glass bottles, which in accordance with the invention have been provided with the coatings and the layers 3-7, are rubbed against one another to enable measuring of the coefficient of friction therebetween. The curves in Figs 4-6, which will be describ¬ ed in the following, are based on such tests. Fig. 4 shows the coefficient of friction for bottles which are provided with SBR and PU coatings and which further have been immersed in a 2% water-based PE solution. At a load of 5 kg on a dry surface, the coefficient of friction is about 0.2, and the corresponding value at a load of 5 kg on a wet surface is slightly below 0.2. These values for the coefficient of friction are much lower than those that can be obtained with an untreated SBR+PU surface, in which case the coefficient of friction has been found to be about 0.3-0.5. Fig. 5 illustrates a similar friction test at a load of 30 kg on dry and wet surfaces with the same coatings. Here, the coefficient of fric¬ tion is well below 0.2, which is highly desirable. Fig. 6 shows the coefficient of friction for different numbers of washings of bottles having the same coatings. The curve in Fig. 6 is based on tests simulating a number of refilling cycles including wash¬ ing. The purpose was to test how well the PE coating could withstand several washings in a basic environment. The bottles were immersed in a 3% NaOH solution at a tempe¬ rature of 80°C for 10 min. Then, the bottles were rinsed in a water bath at a temperature of 80°C for 0.5 min, as well as rinsed under running water at a temperature of 60°C for 1.5 min. Thereafter, the bottles were allowed to dry and cool off to room temperature, where¬ upon the coefficient of friction between the bottles was measured. This washing cycle was repeated 10 times, and the friction was mea- sured every time. The friction measurements accounted for in Fig. 6 show that the PE coating is able to withstand repeated washing in lye solution as above without losing its friction-reducing effect. The coefficient of friction is in the range of 0.1-0.2, depending on the load and on whether the surface is dry or wet. Thus, it is evident from Fig. 6 that there is a surprisingly strong bond between the PU layer and the PE coating, due to which the bot¬ tle shows excellent strength properties as well as low friction during several filling and washing cycles. Fig. 7 shows the above-mentioned coatings and layers 3-7, which here have been separated for illustrative purposes. In the Figure, a few typical thicknesses are indicated. In a second preferred variant of the invention, a friction-reduc¬ ing agent is admixed to the polyurethane that is to form the layer 6. This admixture, which is carried out before the application of the PU layer 6, is produced by adding a polyolefin or, which is preferred, a wax or silicone emulsion to the polyurethane, which preferably is based on polyether. In tests, excellent results have been obtained by the addition of a wax emulsion containing ethylene-bis-stearamide. However, the choice of the friction-reducing agent (lubricant, slip agent) is not critical, as long as the aimed-at low coefficient of friction is obtained. As before, a PE coating is applied to the PU layer to which wax has been added. Glass bottles with coatings according to this second variant have been tested as to friction in same way as in the preceding tests (Figs 4 and 5). The results were as follows. Load Coefficient of friction 5 kg dry surface 0.24-0.26 5 kg wet surface 0.14-0.18 20 kg dry surface 0.09-0.10 20 kg wet surface 0.09-0.13 This second variant thus results in a glass bottle with a protec¬ tive coating having precisely the low coefficient of friction that the invention aims at achieving. This invention is applicable to frosted glass bottles, which have been frosted e.g. by the admixture of silicon dioxide to the PU layer, with or without the addition of a wax or silicone emulsion mentioned above. The final PE coating effectively protects the frosted surface. Furthermore, a decorative layer (e.g. epoxy-based) , which e.g. may include text, can advantageously be introduced between the PE coating and the PU layer. In the first variant of the invention describ¬ ed in the foregoing, such a decorative layer (e.g. of ceramic material) may be introduced between the glass surface and the primer coating. Finally, it should be observed that the invention is by no means restricted to the embodiments described in the foregoing, but that several modifications thereof are conceivable within the scope of the inventive concept as defined in the appended claims. In the foregoing, polyethylene has been given as a non-restrict¬ ing example of the polymeric material used in the outer friction-reduc¬ ing coating. Tests have shown that other polymers having the same effect may well be used. Thus, polyolefins, including olefinic homo- polymers as well as copolymers, are especially preferred. The choice of material is not critical, as long as the aimed-at effect is achieved. However, "pure" homopolymers, such as polyethylene (LD as well as HD) and polypropylene, are preferred, primarily for environmental reasons and reasons of cost. It should be emphasised that satisfactory protection as well as enhanced strength (strength-enhancing effect) can be obtained if the PU layer 6 is applied directly to the glass surface, whereupon the PE coating 7 is applied as the final coating. Fig. 8 shows a curve that illu- strates the enhanced pressure strength. A pressure strength enhance¬ ment of about 30% is obtained, as compared with pure glass. Further, coefficients of friction of about 0.2-0.3 are obtained. This variant is applicable to small and light returnable bottles, in which the retentive effect of the SBR layer is not absolutely necessary. Alternatively, the PU layer can be applied to a primer coating.

Claims: CLAIMS 1. A method of producing an article of glass having a protective coating, especially a returnable glass container, comprising the steps of a) applying to the article a primer coating containing metal oxide, preferably stannic oxide, b) applying to the primer coating a coating of a material having a friction-reducing effect, preferably a fatty acid ester, c) applying to the coating of organic material a first aqueous emulsion that contains a first elastic polymeric material, preferably styrene rubber, d) drying the first aqueous emulsion in order to form a first elastic polymer layer, e) applying to the first polymer layer a second aqueous emul¬ sion that contains a second elastic polymeric material, preferably polyurethane, f) drying the second aqueous emulsion in order to form a second elastic polymer layer, g) curing the two polymer layers jointly, h) applying to the second polymer layer a third aqueous emul¬ sion that contains a third polymeric material, preferably a polyolefin, especially polyethylene, and i) drying the third aqueous emulsion in order to form an outer coating that has a friction-reducing effect and is able to withstand repeated washing in lye solution. 2. A method of producing an article of glass having a protective coating, especially a glass container, wherein one or more layers or coatings, of which at least one contains an elastic polymeric material, are applied to the glass article, one layer of the elastic polymeric material forming an outer layer, and a friction-reducing coating able to withstand repeated washing in lye solution being applied to this outer layer, c h a r a c t e r i s e d by admixing a friction-reduc- ing agent to the elastic polymeric material forming the outer layer, to which is applied the friction-reducing coating. 3. A method as claimed in claim 2, wherein the mixture is pre¬ pared by adding a wax or silicone emulsion to the elastic polymeric material forming the outer layer. 4. A method as claimed in claim 3, wherein the friction-reducing agent consists of a wax emulsion containing ethylene-bis-stearamide. 5. A method as claimed in any one of claims 2-4, wherein the elastic polymeric material forming the outer layer consists of poly- ether-based polyurethane. 6. A method as claimed in any one of claims 2-5, wherein the friction-reducing coating is produced by applying an aqueous emul¬ sion containing a polyolefin, preferably polyethylene, to the outer layer and then drying it. 7. An article of glass, especially a glass container, c h a r ¬ a c t e r i s e d in that it has been produced according to the method claimed in any one of the preceding claims. 8. The use of a wax or silicone emulsion, preferably containing ethylene-bis-stearamide, as a friction-reducing agent which is admix¬ ed to an elastic polymeric material, preferably polyurethane, which is to form a protective coating on an article of glass and to which is applied an additional friction-reducing protective coating, preferably of a polyolefin, especially polyethylene.

Inventors: AUGUSTSSON BENGT, KAWANO NAGAHIRO, AUGUSTSSON, BENGT, KAWANO, NAGAHIRO
Applicants: PLM AB
Application year: 1994
Main classification: C03C17
IPC codes: B65D23, C03C17